The plutonium contamination is intense inside the old plant, where men are working in suits which look as if they were designed for a space walk.

No one knows quite how bad conditions are because the alpha radiation monitoring machine has gone off the dial at 99,000 counts a second - too high to measure. Outside the building in the Cumbrian summer sunshine of Sellafield "normal" levels are five counts a second.

The men are installing robots to cut up plutonium- and uranium-contaminated machinery used in the 1970s and 80s to make fuel for the long disused Dounreay experimental fast breeder reactor in Scotland.

After decades it is suddenly an urgent problem. The plutonium is decaying into another even more dangerous substance, americium. Unlike the plutonium radiation, which cannot penetrate the suits, the gamma dose from the americium goes straight through into the human body.

In charge of the project is 29-year-old Jeremy Hunt, a mechanical engineer, who joined British Nuclear Fuels as a graduate trainee and finds the problem of dismantling this highly contaminated plant fascinating. Among other things, he has to juggle the workload of the men inside the facility so that they do not exceed the maximum permitted legal dose of radiation in any one year.

In theory there is enough plutonium inside the machines still to be taken apart to go critical - that is, cause a nuclear meltdown. Security is tight; it is a category A facility and no journalist has been inside before.

"This is one of the most dangerous jobs anyone could tackle; in theory there is still enough plutonium in there to cause a criticality accident, but we are reasonably confident. We are not having any exercises today, so follow me if the alarms go off," Mr Hunt said cheerily.

"What makes this job so interesting is that this is a fantastic problem-solving job, every bit is different and requires new techniques, specially adapted and built robots, because we cannot expose men to this level of radiation."

BNFL began working on the disman tling of B277 building 10 years ago and has only now got to the difficult bit. Mr Hunt hopes to have the problem solved by the time he is 33, in 2007.

But B277 is just one of Sellafield's problems. There are 400 buildings on four square miles of the site which began life as a TNT factory and evolved into a nuclear bomb factory.

Not far from B277 is B30 - a large open air pond which leaks radioactive water through the joints when the cold weather shrinks the concrete . It is having a secondary containment bund built round it just in case a small earthquake is enough to split it open and cause a major nuclear spillage. The place is rotting away and almost everything is being replaced in order to dismantle it safely.

BNFL is trying to work out how to prevent seagulls landing on the pond. About five a day do so on average, unwittingly carrying away radiation which they subsequently distribute.

Leaning over the pond, peering into the murky depths, while Mike Travis, head of ponds retrieval, points out spent nuclear fuel rods dumped there, he becomes aware that the radiation monitor on my chest is ticking rather fast. "Perhaps," he says, "we should not stand here too long."

Outside, between B30 and the next building, are additional lead shields to prevent people walking past getting a radiation dose through the metre-thick walls. The highest radiation comes from a defuelling bay used as a dump in the 70s. "In the miners' strike we were told to run the Calder Hall reactors at full pelt to keep the lights on, so people got a bit cavalier about the rubbish."

Clean-up bill

For years, Sellafield has been a byword for secrecy and economy with the truth about the waste legacy. At last the reality has caught up with the company - and the taxpayer - in the form of £41bn's worth of liabilities, around £30bn of it at Sellafield.

In effect, the company became technically bankrupt, forcing the government to step in. The Department of Trade and Industry is creating a nuclear decommissioning authority to tackle the problem. BNFL hopes to become the management company to carry on the clean-up, which will run for the rest of this century. In total, including the Ministry of Defence's so far unpublished problems, the taxpayer faces a clean-up bill of £100m, a figure never previously disclosed.

The legacy created by the cold war bomb effort, BNFL, and the rush to import used nuclear fuel from around the world to earn foreign currency, cannot be separated. The largest single nuclear inventory in Europe is in another building, B215, known also as the high-level evaporation and storage facility. High-level nuclear waste is still pumped in from the soon to be redundant nuclear fuel reprocessing works and reduced in volume by boiling in a semi-vacuum.

The new section was opened in 1991 by Michael Heseltine, then environment secretary, but what Stephen Wilson, the plant controller, calls the "old side" dates from the 40s. The nuclear installation inspectorate is worried about lack of progress in dealing with this plant and has ordered a progressive reduction of the 1,400 cubic metres of highly radioactive liquor.

B215 is the Sellafield plant that most frightens the Irish government. Since the September 11 attacks the Irish have worried about the tanks as a target and the danger of a radioactive cloud drifting across the Irish sea.

Security at Sellafield is now much tighter; armed police have been supplemented with many other deterrents and the car park once close to these buildings has been moved. The Guardian was banned from taking photographs in certain areas.

All this has led to a new urgency to get the waste dealt with. The solution to the high-level waste problem is to mix it with glass and turn it into blocks, which can be kept cool in stores until a final disposal route is found. The problem was thought to have been solved 10 years ago when the first vitrification plant was started but the intense radiation from the waste destroyed the machinery which was turning it into glass. The battle to make the technology work efficiently continues, but BNFL is confident that it is winning.

The company's final problem is what to do with all the nuclear junk once it has been safely packaged. Currently, there are hundreds of drums of waste in store, soon to be joined by thousands more, around 50,000 cubic metres. So far the government has no plan for dealing with it. The options include storing it in concrete bunkers above ground or a deep depository underground - the big headache for ministers though, is where.

Weapons and waste that made site infamous

· Windscale, a closed second world war TNT and munitions factory, is taken over in 1947 by the ministry of supply as site suitable for nuclear weapons factory

· First plutonium is produced in 1951 in prototype nuclear fuel reprocessing plant and shipped to Aldermaston, Berkshire, to make bomb for first UK atomic test n 1952

· UK atomic energy authority forms in 1954 to take over site

· Queen opens world's first nuclear power station in 1956, at Calder Hall on Windscale site. It has four Magnox type reactors

· Disastrous fire in 1957 at Windscale Pile No 1, the nuclear plant used for making plutonium.

· Radioactive elements are sprayed over the countryside in the world's worst nuclear accident before Chernobyl. For months all local milk is poured down drain

· Reprocessing works for spent fuel from Magnox reactors opens in 1964 to dissolve nuclear fuel into uranium and plutonium

· Windscale advanced gas cooled reactor opens in 1963, the first of a family of reactors now operated by British Energy. This prototype closes in 1981

· British Nuclear Fuels is formed in 1971 to take over running of Windscale

· In the 70s, first opposition to nuclear power begins and there is widespread protest at nuclear discharges into Irish sea

· Name of plant changed from Windscale to Sellafield to "improve" image

· Inquiry into leukaemia cluster at Sellafield, identified in early 80s, fails to pinpoint cause

· Government abandons plans in 1988 for a fast breeder reactor programme using plutonium fuel as it is "uneconomic"

· Thorp reprocessing plant opens in 1994

· Hours before the 1997 election, plans for a deep depository for nuclear waste under Sellafield abandoned by Conservative government